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Coolant problem

Recently came down the Thames form Teddington to Limehouse and was punching the tide for the first hour or so with my Canaline 42hp engine running at 1,600rpm. Engine temperature gradually increased from low 70's to mid 80's and stayed there for remainder of trip decreasing after the tide turned to mid 70's and as revs were decreased to more normal 1,500rpm cruising speed. On arrival at Limehouse, was a large amount of clear glycol mix in the sump pan yet the header tank has not lost any coolant. Could an air lock in the skin tank be the problem? Can't see any obvious leaks and running for power for last couple of days in Paddington basin has seen temperature constant in low 70's as usual. Suggestions?

Asked by: peter osullivan  | 9.07pm, Thursday 11 September

WW says:

An air lock does sound most likely which could cause localised overheating. If the temperature rises above a certain level, the pressure will increase in the system until the pressure relief can vents coolant out from the pressurised side of the coolant system. Usually the Canaline 42 has a small clear hose leading from the pressure cap into the sump catch all- which is presumably where the coolant has gone. Bleeding the pipework and ensuring all joints in the cooling pipework are tight and not leaking should ensure no more air comes in. However, tap water in particular, but also ready-mix collant, has air dissolved in it, which will be driven out of the system over time- and it doesn't always make it's way to the header tank. Purging the skin tank (hopefully it has a bleed screw) regularly help- and if the calorifier pipework is higher than the engine, that can airlock sometimes.
There are a few other possible causes, which you might want to check. Sudden increased loading, such as fouled prop, can cause the engine to overheat, without the refs noticeably dropping- plastic bags on the shaft, with friction against the aft shaft bearing, can be a problem..
There is also a good chance that the skin tank is undersized for the engine- a minimum size of 1sq ft per 4 HP is a good minimum side, assuming it has internal baffles to prevent short circuiting- it needs to be bigger if a simple slab tank with no battles. When you run even a little above normal revs, as well as potentially displacing any air trapped in the system, the thermal load is substantially greater. Also, punching a tide, the engine might be running at day 1600RPM, but the energy load required to turn the prop against the current might be, say, closer to 1800 or 2000rpm. Think about going uphil in a car- to keep at the same speed (and engine revs) you have to push further down on the throttle- burning more fuel and so releasing more heat. The same applies to boats, but is just less apparent
If the engine cannot dissipate the heat, then the coolant temperature will rise and then the pressure will go up, tripping the expansion cap on the engine.
It may be worth measuring your skin tank. If it is not around 10sq ft for your engine, and you intend to do lots of tidal work, then consider fitting something else to dump the thermal load- like a heat exchanger into the central heating system, or even a car heater in the calorifier return.

Mark Langley  | 9.29PM, Thursday 11 September

It could be that the fluid is not mixed properly-
Or that you have had steam (water vapour) vent off from the pressure cap- which would leave the antifreeze behind. Take off the engine header cap and test the fluid there- it should be pink. If not, the fluid is definately not mixed properly, with the antifreeze and water seperate. However, my money would be on having steam vent off the header cap.

Mark Langley  | 9.30PM, Friday 12 September

Readers say:

Strangely, my header tank and bleed screw both produce pink fluid whereas the coolant in the sump pan is clear? Any idea why?

peter osullivan  | 5.12PM, Friday 12 September

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